Morbidity and mortality due to infections remain serious problems in pediatric oncology patients receiving chemotherapy. Association of mannose-binding lectin (MBL) levels with an increased risk for infection in previous studies was contradictory. The aim of this study was to determine whether MBL deficiency is associated with the risk of infections in pediatric oncology patients. Before the start of chemotherapy a blood sample was taken from 75 patients with acute lymphoblastic leukemia and MBL serum concentration was measured using a commercially enzyme-linked immunosorbent assay kit. Twenty patients had concentrations under 1000 µg/L, defining MBL deficiency and the remaining 55 patients had concentrations >1000 µg/L. Ten patients suffered from more than 1 episode of severe infection. Sixty-five percent of patients with MBL below 1000 µg/mL suffered from 2 or more episodes of infections (3 of 16 individuals with 1 severe infection; 10 of 16 with 2 and 3 of 16 with 3), in contrast to only 29 of 55 (52%) patients with MBL above 1000 µg/mL (19 of 27 individuals with 1 severe infection and 10 of 27 with 2). The difference between 2 groups was significant (P<0.001). The results of this study indicate that low MBL serum levels (<1000 µg/L) identify pediatric cancer patients at increased risk for infections.